This command reads the next line from channelId, returns everything
in the line up to (but not including) the end-of-line character(s), and
discards the end-of-line character(s).

ChannelId must be an identifier for an open channel such as the
Tcl standard input channel (stdin), the return value from an
invocation of open or socket, or the result of a channel
creation command provided by a Tcl extension. The channel must have
been opened for input.

If varName is omitted the line is returned as the result of the
command.
If varName is specified then the line is placed in the variable by
that name and the return value is a count of the number of characters
returned.

If end of file occurs while scanning for an end of
line, the command returns whatever input is available up to the end of file.
If channelId is in nonblocking mode and there is not a full
line of input available, the command returns an empty string and
does not consume any input.
If varName is specified and an empty string is returned in
varName because of end-of-file or because of insufficient
data in nonblocking mode, then the return count is -1.
Note that if varName is not specified then the end-of-file
and no-full-line-available cases can
produce the same results as if there were an input line consisting
only of the end-of-line character(s).
The eof and fblocked commands can be used to distinguish
these three cases.